DEVELOPMENT OF ELASMOBRANCH FISHES. 123 



the diverticula after the separation of the tuater-va^cidar' system, 

 forms the hody-cavity. The outer lining layer of the cavity forms 

 the somatic layer of mesoblast and the voluntary muscles ; the 

 inner lining layer the splanchnic mesoblast which unites luith the 

 epithelium of the alimentary tract. Though this fundamental 

 arrangfement would seem to be universal amons^st Echinoderms, 

 considerable variations of it are exhibited in different groups. 



There is one outgrowth from the alimentary tract in Sy- 

 napta ; two in Echinoids, Asteroids and Ophiura ; three in 

 Comatula, and four (?) in Amphiura. The cavity of the out- 

 growth usually forms the body-cavity, but sometimes in Ophiura 

 and Amphiura (MetschnikofF) the outgrowths are from the first 

 or soon become solid, and only secondarily acquire a cavity, 

 which is however homologous with the body-cavity of the other 

 groups. 



In Saoitta* the formation of the mesoblast and the ali- 



o 



mentary tract takes place in nearly the same fashion as in the 

 Echinoderms. The simple invaginate alimentary cavity becomes 

 divided into three lobes, a central and two lateral. The two 

 lateral lobes are gradually more and more constricted off from 

 the central one, and become eventually quite separated from it; 

 their cavities remain independent, and form in the adult the 

 hody-cavity, divided by a mesentery into two distinct lateral 

 sections. The inner layer of each of the two lateral lobes forms 

 the mesoblast of the splanchnopleure, the outer layer the meso- 

 blast of the somatopleure. The central division of the primitive 

 gastrsea cavity remains as the alimentary tract of the adult. 



The remarkable observations of Kowalevsky^ on the devel- 

 opment of the Brachiopoda have brought to light the unex- 

 pected fact that in t\vo genera at least (Argiope and Tere- 

 bratula) the mesoblast and body-cavity develope as paired 

 constrictions from the alimentary tract in a manner almost 

 identically the same as in Sagitta. 



It thus appears that, so far as can be determined from the 

 facts at our disposal, the mesoblast in almost all cases is derived 



1 Kowalevsky, Wxirmer u. Artliropoden, Mem. Acad. Pctersbonrg, 1871. 



2 Zur Entwickluugsgeschichte d. Brachiopoden Protokoll d. Ersten Session der 

 Versammlung Eussischer Naturforscher i. Kasan, 1873. Published in Kaiser- 

 liche Gesrllschaft Moskau, 187-4 (Russian). Abstracted in Hoffmann and 

 Scbwalbe, Jahreshericht f. 1873. 



